Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next?

Given data

• Series: 21, 9, 21, 11, 21, 13, 21, ...

Concept/Approach
Alternating constant 21 with an increasing odd sequence (9, 11, 13, 15, ...).

Step-by-Step calculation
After 21, the odd numbers go 9 → 11 → 13 → next 15So the next term following 21 is 15

Verification/Alternative
Two interleaved sequences: positions odd = 21; positions even = 9, 11, 13, 15, ...

Common pitfalls

• Expecting a single arithmetic progression.

Final Answer
15